1. Field of the Invention
The present invention generally relates to an light sensing circuit and control method thereof. More particularly, the present invention relates to a light sensing circuit capable of voltage compensation and the control method thereof.
2. Description of the Related Art
Referring to FIG. 1, the circuit diagram of a conventional light sensing circuit 9 is shown. The light sensing circuit 9 includes a photo transistor 91 and a control transistor 92, and these two transistors 91, 92 can be thin-film transistors (TFTs). The gate and source of the photo transistor 91 electrically connect to a light sensing scan signal “Vs1,” the bulk of the photo transistor 91 electrically connects to a bias voltage “Vb,” and the drain of the photo transistor 91 electrically connects to a capacitor 93. The source of the control transistor 92 also electrically connects to the capacitor 93, the gate of the control transistor 92 electrically connects to a control scan signal “Vs2,” and the drain of the control transistor 92 alternatively connects to a zero signal “Vm,” such as a zero volt signal, or a output buffer 95 via a switch 94. The buffer can be adapted to output a light sensing signal “Vo” so as to be widely applied to various electrical equipments, such as digital camera, touch screen, etc.
Referring to FIG. 2, a control sequence chart of the conventional light sensing circuit is shown. According to both of FIGS. 1 and 2, there may be a preparation stage “P0” before the light sensing circuit 9 operates, wherein the control transistor 92 can connect to the output buffer 95 via the switch 94, the light sensing scan signal “Vs1” and control scan signal “Vs2” can be predetermined at a negative voltage level, such as 0 volt, and the bias voltage “Vb” can be negative for the photo transistor 91 to work under a negative bias. Thereafter, a sensing stage “P1,” a readout stage “P2” and a global reset stage “P3” are sequentially carried out. In the sensing stage “P1,” a light sensing process is performed, wherein the light sensing scan signal “Vs1” can be set at a positive voltage level, such as 5 volts, and the photo transistor 91 is not turned on due to the negative bias. However, if the photo transistor 91 is exposed to light, such as UV light, a detection current “I,” namely a positive potential, will charge the capacitor 93 to the positive voltage level. In the readout stage “P2,” a readout process is performed, wherein the light sensing scan signal “Vs1” can be set at the negative voltage level for maintaining the voltage “Va” of the capacitor 93 at the positive voltage level. The control scan signal “Vs2” can be set at the positive voltage level to turn on the transistor 92, so that the voltage “Va” is outputted as the light sensing signal “Vo.” In the global reset stage “P3” the control scan signal “Vs2” can be kept at the positive voltage level, while the source of the control transistor 92 connects to the zero volt by switching the switch 94. Therefore, the voltage “Va” of the capacitor 93 can be discharged through the control transistor 92 and the switch 94. With the above stages, the conventional light sensing circuit 9 may detect the illumination of the surrounding repeatedly.
However, Please refer to FIGS. 3a and 3b, which shows, under different threshold voltages “Vth,” the current curves and the voltage curves of the UV-light-induced detection current “I” and voltage “Va” of the conventional light sensing circuit. The “Vg−Vth” indicates the voltage difference between the gate voltage “Vg” and the threshold voltage “Vth.” The “U1,” “U2,” and “U3” indicate the current curves of the photo transistor 91 with threshold voltages of 2 V, 4.5 V and 7 V. The “U4,” “U5” and “U6” indicate the voltage curves of the photo transistor 91 with threshold voltages of 3 V, 5 V and 7 V. Specifically, since the light sensing circuit lacks of voltage compensation function, the threshold voltage “Vth” of light sensing circuit may be different from an ideal value due to manufacturing tolerance or aging. Furthermore, according to FIGS. 3a and 3b, the larger the threshold voltage “Vth” is, the smaller the detection current “I” is, and the difference between two voltages “Va” respectively corresponding to two threshold voltages “Vth” is also large. Therefore, the reliability of the conventional light sensing circuit 9 may be low due to unstable detection results or errors in determining illumination, which are resulted from differences in light induced current decay or sensitivity.
Thus, a need to improve the above technique exists when applying it to practical use.